Heretofore, two modes i.e. a dynamic scattering (DS) mode and a phase change (PC) mode are known for liquid crystal optical devices utilizing light scattering as the operational principle. In the DS mode, a liquid crystal material having a conductive substance incorporated therein and having a nagative dielectric anisotropy is sealed in a pair of substrates treated for homogeneous or homeotropic alignment and provided with transparent electrodes, and it is designed to control two states i.e. a transmitting state when no voltage is applied and a state where a voltage higher than the threshold voltage is applied to cause dynamic scattering and thereby to reduce the transmittance. Whereas, in the PC mode, a cholesteric liquid crystal material is sealed in a pair of substrates provided with transparent electrodes, which are optionally treated for alignment, and it is designed to control two states i.e. a state of a neumatic phase in homeotropic alignment (transmitting) and a state of a cholesteric phase in focalconic or planar alignment (scattering) by the application or non-application of a voltage. Both the DS and PC modes have an advantage that a wide viewing angle is obtainable since no polarizing film is used in either mode. However, the DS mode has a drawback that since it is a current effect mode with a conductive substance added to the liquid crystal material, the power consumption is large and the reliability of the liquid crystal material is low. The PC mode has also a difficult problem such that since the operation voltage depends upon the ratio of the cell gap to the pitch of liquid crystals, a highly precise uniform gap is required to enlarge the display surface area.
On the other hand, H. G. Craighead et al. have disclosed in Appl. Phys. Lett., 40(1)22(1982) a method which utilizes the feature that liquid crystal has birefringence. Specifically, the liquid crystal is impregnated in a porous material, and by the application or non-application of an electric field, the refractive index of the liquid crystal is changed, whereby the transmittance and the scattering are controlled by adjusting the difference in the refractive index from the porous material. This method is useful in that the drawback of the DS and PC modes can be overcome in principle without employing polarizing films. Similar devices have been prepared by J. L. Fergason et al. who used nematic liquid crystal encapsulated by means of polyvinyl alcohol (U.S. Pat. No. 4,435,047), by K. N. Pearlman et al. who used liquid crystal having various latexes taken therein (European Patent Application Publication No. 156615), and by J. W. Doane et al. who used a method of dispersing liquid crystal in an epoxy resin, followed by curing (International Application Publication No. 85-4262).
The method of H. G. Craighead et al. had drawbacks that no adequate variation in the transmittance is obtainable and the preparation of a device is difficult because it employs a process of impregnating liquid crystal into a porous material and thus has problems such that the impregnation of the liquid crystal is difficult when there is irregularities in the size of pores of the porous material used and the proportional ratio between the porous material and the liquid crystal can not freely be selected. Likewise, the devices of J. L. Fergason et al. and K. N. Pearlman et al. had a drawback that they require patterning of electrodes for a display. This patterning of electrodes require many steps including coating of a resist on the electrodes, exposure to light, development and etching. Besides, this display was limited to a two value display (display by two levels of transmittance) of transmitting and not-transmitting as between a portion where electrodes face each other and a portion where no electrodes face each other. Further, it was impossible to display a character or design in a circular or rectangular frame. The device was of a type where the display is on only when a voltage is applied.